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Remove items from custom arraylist implemetation without .remove

I'm working on a custom ArrayList implementation and I have one method where I'm trying to remove an item per conditions from an array such as E[] elements. The array is initialized by doing something like this:
String[] contents = {"chicken", "hippo", "goat"};
ArrayI<String> newarray = new ArrayI(contents);
newarray.chooser(new LongChooser());
It should remove words length 4 or less and return an array like this:
["chicken", "hippo"]
I'm trying not to use any built in methods, like remove(), clone(), arraycopy(), etc. I can't seem to get this to work, I've tried creating a duplicate array and trying to copy elements over like this:
E[] copy = (E[]) (new Object[this.size-1]);
for (int i = 0; i < size; i++) {
if (shorter) {
copy[i] = elements[i];
}
else {
for (int j = i; j<this.size-1; j++) {
elements[j] = elements[j+1];
}
elements[size-1] = null;
size -= 1;
}
for (int i =0; i< copy.length; i++) {
elements[i] = copy[i];
}
size -= 1;
I know this is not the correct way because they aren't the same size array and just returns [longword, longerword, null]. Also I'm pretty sure I should be using the size variable, but it doesn't seem to do much.
How do I get this to work? Thanks.

Create an array to hold the [filtered] results. Its initial size is zero.
Iterate through contents.
If the current element of contents needs to be retained, then
create a temporary array whose length is one greater than the array that holds the results.
copy the results array to the temporary array
set the last element of the temporary array to the current element of contents
assign the temporary array to the results array
Here is the code, using only simple arrays. I presume you can adapt it to your needs. Note that the last line is simply to check the value of newContents. It is not required.
String[] contents = {"chicken", "hippo", "goat"};
String[] newContents = new String[0];
for (String str : contents) {
if (str.length() > 4) {
String[] temp = new String[newContents.length + 1];
for (int i = 0; i < newContents.length; i++) {
temp[i] = newContents[i];
}
temp[newContents.length] = str;
newContents = temp;
}
}
System.out.println(Arrays.toString(newContents));

How do I combine the first element of 2 array together into a string array?

String quantityArray[] = GetStringArray(quantity);
String foodItemArray[] = GetStringArray(fooditems);
this is to change from ArrayList to a String Array
int n1 = fooditems.size();
int n2 = quantity.size();
for(int s = 0; s<n1;s++){
totalFood[s] = quantityArray[s] + foodItemArray[s];
}
I cannot make this totalFood[] function to work as it just keeps crashing my app
public static String[] GetStringArray(ArrayList<String> arr) {
// declaration and initialise String Array
String str[] = new String[arr.size()];
// ArrayList to Array Conversion
for (int j = 0; j < arr.size(); j++) {
// Assign each value to String array
str[j] = arr.get(j);
}
return str;
}
The error that pops up is (Attempt to write to null array)

You need to make sure totalFood array is allocated.
Arrays are themselves Objects in Java.
For example:
totalFood = new String[n1];
This is because totalFood seems to be null according to the error you are seeing. You need to allocate some space (n1 references to be precise for the above example) for the Strings resulting from the concatenation to be stored at. You can take a look at the corresponding Java tutorial here.
Also make sure n1 is equal to n2 which should be equal also to the size of the totalFood array in order to not overrun any of them in your loop.
Finally, there are 2 handy Collection#toArray methods which will do what you are doing in your GetStringArray method. You can use it for example like so:
String quantityArray[] = quantity.toArray(new String[quantity.size()]);
Using the toArray method seems not to be related to the problem, but I just mention it for completeness.

public static String[] GetStringArray(ArrayList<String> arr) {
String str[] = new String[arr.size()];
for (int j = 0; j < arr.size(); j++) {
str[j] = arr.get(j);
}
return Arrays.stream(str).filter(s -> s!=null).toArray(String[]::new);
}

Java Null Pointer with String Array [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 5 years ago.
I have been coding a basic method to take an array of strings, find the largest length and return a string array containing on values that are equal to the highest length of the array. I keep getting a null pointer exception and i am not sure why. The code is:
String[] allLongestStrings(String[] inputArray) {
int compare = 0;
int k = 0;
String[] use = new String[20];
for (int i = 0; i < inputArray.length; i++) {
if (inputArray[i].length() > compare)
compare = inputArray[i].length();
}
for (int j = 0; j < 20; j++) {
if (inputArray[j].length() - compare == 0) {
use[k] = inputArray[j];
k++;
}
}
return use;
}

for (int j = 0; j < 20; j++) {
if (inputArray[j].length() - compare == 0) {
use[k] = inputArray[j];
k++;
}
}
This will only work if inputArray has at least 20 elements. In the code above, you're doing the correct thing: for (int i = 0; i < inputArray.length; i++). I think you just need to change this second for statement to be the lesser of 20 or the length of inputArray.

inputArray doesn't contain 20 elements. Don't just throw out and hard code a length value for your the array you're going to return. Actually determine what the true length is going to be because you could be too low on that value and if your not then you could end up with a bunch of null elements.
If allowed use an ArrayList or String List object which doesn't require a preset length (size) instead of a 1D String Array which does require length initialization: List<String> list = new ArrayList<>();. You can simply just add to it with the List.add() method.
If you must use a Single Dimensional Array then use another for loop to determine how many string elements actually have the same length as what is held within the compare integer variable. Always iterate through the original array (inputArray). This for loop would be very much like your second for loop except you would increment a integer counter variable upon all elemental lengths that equal the value held within the compare variable.
Eliminate the formula in your if statement condition contained within your second for loop. I think (IMHO): if (inputArray[i].length() == compare) {...} should be sufficient, it's much easier on the eyes. ;)
Just a thought for pizazz....perhaps add the actual Array index to the string that is added to the 1D String array named used. Use a delimiter of some sort to separate the two.

Java method to multiply an ArrayList of decimal values by an array of doubles to get an array of ints?

Help! I have this school assignment that wants me to write a method to find the area of rectangles (an array of ints) by multiplying width (an ArrayList) by length (an array of doubles). I'm very very new to coding; I've tried for over five hours to get this working, but I keep doing things wrong and I simply can't get it right. This is the code for the method that I've written:
public void calcRectangleArea(int index, ArrayList width, double[] length, int[] area)
{
double temp = length[index];
for(index = 0; index < length.length; index++)
{
for(index = 0; index < width.size(); index++)
{
Object widthObj = (int)width.get(index);
area[index] = temp * widthObj;
}
}
}
The full starter code we were given is here, if you need more context (it's commented): http://pastie.org/pastes/916496
Thank you so much for any help you can give me in writing this method. I've been working for hours and I just can't get it...

The length of the array and size of the arraylist should be same , And you have to change the method logic a bit, Have a look at the below code snippet
public static int[] calcRectangleArea(List<Double> width, double[] length)
{
int[] area=new int[length.length];
for(int index = 0; index < length.length; index++)
{
area[index] = (int) (length[index]*width.get(index));
}
return area;
}
Call this method passing width arraylist and length array. It will return area array of ints

You dont really need two loops here. Assuming that width[1] correlates to length[1] you can just loop through both collections at the same time in the same loop.
This should work (i haven't written a line of java in ~2 years so it may not be 100%)
public void calcRectangleArea(int index, ArrayList width, double[] length, int[] area)
{
//assuming length.length == width.size
for(index = 0; index < length.length; index++)
{
int anArea = (int)(width.get(index) * length[index]);
area[index]=anArea;
}
}
again the code above assumes the size of the collections are the same.

Firstly, you don't need to assign temporary variables:
double temp = length[index];
...
Object widthObj = (int)width.get(index);
Since you will only reference them once. Reference them directly instead:
area[index] = length[index] * (int)width.get(index);
Secondly, your for loops are unneeded, and they are declared wrong. You're trying to increment the index (and twice nonetheless) that was passed to the function which will cause problems. If you were to use nested for loops, you would declare a new iterator variable for each of them:
for (int i = 0; i < something; i++) {
for (int j = 0; j < somethingElse; j++) {
doSomething();
}
}
However in this case you don't even need them.
In addition, you should not have created an Object when you wanted to cast an int:
Object widthObj = (int)width.get(index);
should have been
int width = (int)width.get(index);
However again, this line is unnecessary, and you shouldn't be casting to int this early.
Ultimately, all you need to do is the one line:
area[index] = (int)(length[index] * width.get(index));

Nullpointerexception with java bubble sort

I have been working on a project for way to long now and I am running into a nullpointerexception. I understand that it is when an object is pointing to nothing. I am getting this error while doing a bubble sort in Java. I can't figure out what is causing this exception and thus can't resolve it. The purpose of this code is to sort an array of student ID numbers in a specific order, I've chosen descending order.
public static void idNumber()
{
String[] iD = new String[150]; //array for ID Numbers
//System.out.println("Original order");
for(int i = 0; i < nNumStudents; i++) //add ID numbers to array iD
{
iD[i] = srStudents[i].getStudentKey();
//System.out.println(srStudents[i].getStudentKey());
}
//bubble sort
int k =0;
int j =0;
boolean exchange = true;
String temp;
temp = new String();
while ((k < iD.length - 1) && exchange)
{
exchange = false;
k++;
for(j = 0; j < iD.length - k; j++)
{
if(iD[j].compareTo(iD[j + 1]) > 0)
{
temp = iD[j];
iD[j] = iD[j + 1];
iD[j + 1] = temp;
exchange = true;
}
}
}
System.out.println(iD);
}
Exception in thread "main" java.lang.NullPointerException
at java.lang.String.compareTo(String.java:1139)
at StudentRegistrar.idNumber(StudentRegistrar.java:152)
at Sort.main(Sort.java:21)

From a glance at your code, my guess is that it is possible that your array size exceeds the number of students. If this is the case, you are attempting to compare empty slots in the array, which would give a null pointer exception. To fix this, increment to nNumStudents rather than to the full length of the array.

This nullpointer is coming up because all the members of String array String[] iD = new String[150]; are not initialize for example the for loop which is populating this iD array is either not running until 150 or one of its members is initialized with null so
First thing print and check what is the value of nNumStudents it should be 150. Then make sure that every value which is assigned to iD array is a non null value you can do this by modifying your code to print all the values it is assigned to
for(int i = 0; i < nNumStudents; i++) //add ID numbers to array iD
{
iD[i] = srStudents[i].getStudentKey();
//uncomment the below line and see if it doesn't print null
System.out.println(srStudents[i].getStudentKey());
}
if it exceeds 150 then you will get an ArrayIndexoutofbound exception not null pointer